1. Field of the Invention
The present invention relates generally to a distance measuring apparatus used with an image pickup system for various types of cameras, such as video cameras and still cameras. More specifically, the invention relates to an active type of automatic focus detecting device arranged to project spot light such as infrared rays onto an object, receiving reflected light by at least a pair of light receiving elements, and obtaining a signal representative of a detected state of focus.
2. Description of the Related Art
With reference to FIGS. 1 to 4(b), description will be made below of a typical form of construction of a so-called active type automatic focus adjustment system in which focus is adjusted on the basis of the difference between the outputs supplied from a pair of light receiving elements.
As shown in, for example, FIG. 1, when a light emitting element 2 is driven by a pulse generated by a pulse generator 1, the light emitting element 2 projects spot light onto an object S. The light reflected by the object S enters a pair of light receiving elements 4a and 4b through a converging lens 3. As an image pickup lens (not shown) is moved for focus adjustment, the light receiving elements 4a and 4b are capable of moving in the directions of a double headed arrow shown in FIG. 1. The outputs of the light receiving elements 4a and 4b are respectively delivered through amplifiers 5a and 5b to synchronizing detector circuits 6a and 6b which are synchronized with a pulse generator 1. The synchronizing detector circuits 6a and 6b remove unnecessary noise components such as external light from the inputs, and supply them to the following integrators 7a and 7b.
As shown in FIG. 2(a), the levels of outputs Sa and Sb of the integrators 7a and 7b gradually increase with the passage of time t. As an example, when Sa&lt;Sb in which the intensity of the spot light incident on the light receiving element 4b is far stronger than that of the spot light incident on the other element 4a, a camera (not shown) is out of focus. On the other hand, if the camera is in focus, difference is not substantially found between the levels of the outputs Sa and Sb, and thus Sa.apprxeq.Sb is obtained. The outputs Sa and Sb of the integrators 7a and 7b are input to an adder 8 for further adding of these outputs Sa and Sb, and at the same time to a substracter 9 for finding the remainder between the absolute values of the outputs Sa and Sb. The outputs of the adder 8 and the subtracter 9 are respectively input to comparators 10 and 11 each having a set of predetermined threshold values V1 and V2 (V1&gt;V2). The outputs of the comparators 10 and 11 are connected to a processing circuit 12.
Referring to FIG. 2(b) showing the output of the adder 8, the level of an output Sa+Sb gradually increases with the passage of time t, and, at a time t1 at which a threshold value V1 is reached, the comparator 10 outputs an "H" level.
Referring to FIG. 2(c) showing the output of the subtracter 9, the level of an output .vertline.Sa-Sb.vertline. gradually increases with the passage of time t, and, at a time t2 at which a threshold value V2 is reached, the comparator 11 outputs the "H" level. The processing circuit 12 uses these "H" levels in order to determine which of the times t1 and t2 is input to the circuit 12 earlier or how much earlier either of them is input thereto.
For example, where the time t2 at which .vertline.Sa-Sb .vertline.=V2 is supplied earliar than the time t1, the relationship of Sa=Sb is not created, that is, the lens (not shown) is out of focus. Conversely, where the time t1 at which Sa+Sb=V1 is supplied earlier than the time at which .vertline.Sa-Sb.vertline.=V2, it can be judged that a state close to Sa.apprxeq.Sb is created and the lens is close to the focused state.
U.S. Pat. No. 4,389,106 disloses a technique by which the integrated outputs Sa and Sb of two light receiving elements are individually compared with a predetermined level, and operations are performed on the thus-obtained results.
However, the above-described conventional active type of automatic focus detecting device involves the following disadvantages.
As an example, when the quantity of light reflected from an object is at a low level, if unexpected noise enters from a control circuit or other external factors, either or both of the outputs Sa and Sb is greatly varied due to the influence of the noise. Thus, the integrated output Sa, Sb or Sa-Sb containing errors such as noise unavoidably reaches a predetermined threshold value. At this time, unnecessary judgement is made as to whether or not the lens is in focus, thus leading to an unstable focusing operation.
The typical synchronizing detector circuits 6a, 6b and the integrator 7 shown in the circuit diagram of FIG. 1 will be described below with reference to the detailed circuit diagram shown in FIG. 3. A capacitor 13b cuts off dc components from the output of an amplifier 13a, and applies such output to an analog switch 13c. The analog switch 13c and an analog switch 13d are alternately opened and closed in response to a synchronizing signal SYNC. A reference voltage Vo is applied to an input of the analog switch 13d. When the input shown in FIG. 4(a) is input as the voltage Vo, the potential at one end of a resistor 13e shows the waveform shown in FIG. 4(b). The rate of the current flowing in the resistor 13e is porportional to the potential difference between the potential shown in FIG. 4(b) and the voltage Vo. However, this relationship is established when an ideal amplifier is used as an operational amplifier 13f. Actually, a certain level of offset voltage is produced between the positive and negative input terminals of the operational amplifier 13f. This offset voltage is normally added to the input of a Miller integrating circuit constituted by the resistor 13e, the operational amplifier 13f and a capacitor 13g, so that, even if the input of the Miller integrating circuit is "0", the result of integration unavoidably includes a potential difference other than "0". To solve the problem, a voltage which is biased by an amount equivalent to the discrepancy with respect to Vo is applied through a variable resistor 13h to the positive input terminal of the operational amplifier 13f, and thus a manual adjustment is made so that the integral result relative to a zero input reaches "0". However, since the offset amount is varied due to the influences of external factors such as temperature changes, this prior-art adjustment also entails the risk of making the accuracy of distance measurement unstable.
Incidentally, U.S. patent application Ser. No. 603,660 discloses related art to the present patent application.